Cellulosic fibres are separated from wood or from other plant materials such as straw and bagasse, by a pulping process such as kraft or sulphite digestion. The resulting pulp still contains a significant amount of lignin and is generally dark coloured. In order to form pulp suitable for paper, a bleaching process is conducted on pulp slurries to remove the residual lignin, in other words, to delignify the pulp, and also brighten the dark coloured pulp.
Conventional bleaching processes such as CEDED, where C stands for chlorine bleaching, E for caustic extraction and D for chlorine dioxide, has in the past been practised by the pulp and paper industry. This process achieves certain brightness levels of pulps required for paper making. However, the use of chlorine and chlorine contained chemicals as bleaching agents is now considered to be environmentally unacceptable because large amounts of chlorinated organic materials are produced in the bleaching processes and difficulties arise in disposing of the used bleaching liquids.
In order to achieve the required brightness levels of pulps and eliminate the formation of chlorinated organics in the chemical pulp bleaching process, other chemicals than chlorine, chlorine dioxide or chlorine based chemicals are required as bleaching agents. Presently bleaching methods based on the use of oxygen, hydrogen peroxide and ozone have been developed and partially used in practice as a replacement for the chlorine based chemicals. Advantages of using these oxygen based bleaching chemicals are clearly beneficial from the point of view of environmental concerns, however, there are drawbacks and limitations with these methods which restrict their wide applications in pulp mills. For example, oxygen bleaching and delignification can only be applied to reach 40% to 50% reduction of the residual lignin content in lignocellulosic fibres, beyond which severe degradation of the cellulosic fibres occurs and pulp qualities deteriorate.
One disadvantage of hydrogen peroxide bleaching process is its ineffective action on lignin, even though it is known that hydrogen peroxide is a good brightening agent. If severe bleaching conditions such as high temperature are used in the peroxide bleaching stage, it leads to significant cellulose degradation.
Use of ozone, which is an delignifying agent, also results in severe fibre damage because of its intrinsic poor bleaching selectivity. Another disadvantage in ozone bleaching is that the process is uneconomical due to high capital expenditure for suitable equipment, and high processing costs. Thus, bleaching processes based on these oxygen based chemicals are not economical and do not achieve the same desired pulp qualities as those processes using chlorine based chemicals as bleaching agents.
Use of hydrogen peroxide to bleach chemical pulps, particularly oxygen delignified softwood kraft pulps, has been limited due to its weak bleaching action to remove residual lignin. Therefore, it is an aim of the present invention to provide a much improved hydrogen peroxide bleaching process for chemical pulp bleaching. It is known that increased reactivity of hydrogen peroxide through its conversion to other more reactive peroxy compounds leads to better peroxide bleaching performances. For example, the use of peracetic or peroxymonosulphuric acids, which can be generated from hydrogen peroxide, as a pulp bleaching agent is known.
Organic nitriles are known as activators for hydrogen peroxide or other peroxides. Reference is made to U.S. Pat. No. 2,927,840 to Dithmar et al and U.S. Pat. No. 3,113,951 to Williams et al. It is also known in the art that in textile bleaching, nitrile compounds such as cyanamide or its derivatives have been described as peroxide bleaching activators. Such examples can be found in U.S. Pat. No. 3,756,774 to Kirner et al, U.S. Pat. Nos. 4,025,453 and 4,086,175 to Kravitz et al, U.S. Pat. No. 4,392,975 to Tourdot et al and U.S. Pat. No. 4,559,158 to Hase et al. Various nitriles are disclosed as being suitable for the purpose of peroxide activation, but no indication was disclosed for any given specific nitrile compound being more effective in the peroxide activation. Kirner et al and Kravitz et al (U.S. Pat. No. 4,025,453) both mention dicyandiamide, referred to as dicyanodiamide, being used as an activator for hydrogen peroxide under acidic conditions in the bleaching of textile materials. However, this is but one organic nitrile referred to in the patents and no advantage is shown for using this specific compound as compared to the other organic nitrile compounds. In fact, Kravitz et al demonstrates that the use of dicyandiamide is disadvantageous compared to that of cyanamide.
German Patent No. 4,004,364 to Sturm and U.S. Pat. No. 5,034,096 to Hammer at al both disclose processes for bleaching and delignifying lignocellulosic materials or pulps with peroxides and with activators of cyanamide or its salts. These references show that when cyanamide or its salts are added into the peroxide bleaching process, there is a significant improvement in the bleaching performance of sulphite pulps. Thus, increased delignification and brightness gain were achieved compared to that attained in peroxide bleaching processes without cyanamides.
The hydrogen peroxide bleaching processes where found to be less effective when applied to oxygen delignified softwood kraft pulps (see Sturm in 1993, Non-Chlorine Bleaching Conference) because oxygen delignified softwood kraft pulp is much more difficult to bleach. We have surprisingly found that whereas cyanamide used as an activator in the peroxide bleaching process is an improvement for some chemical pulps, in other cases the cyanamide was not beneficial but rather deteriorated the bleaching performance of hydrogen peroxide. This was particularly true on pulp brightness developments.
It is an object of the present invention to provide a process for delignifying and bleaching chemical pulps, particularly oxygen delignified softwood kraft pulps, with hydrogen peroxide or peroxides and with the use of more effective peroxide activators which avoids the disadvantages of known processes.